This invention relates generally to a system and method for improving air quality and, more specifically to a system and method for substantially preventing fuel vapor emissions that are caused by fuel heating during diurnal temperature variations (commonly referred to as xe2x80x9cdiurnal emissionsxe2x80x9d), engine operation, and refueling.
Air pollution negatively impacts the environment and the health of its inhabitants. One of the sources of air pollution are gases that escape from liquid fuels, particularly gasoline, and form a vapor in a process called vaporization or evaporation. These fuel vapors are collectively referred to as xe2x80x9cevaporative emissionsxe2x80x9d and include diurnal emissions and other evaporative emissions as hereinafter described.
Diurnal emissions arise from the increase in the volatility of the fuel and expansion of the vapor in the fuel tank due to the diurnal rise in ambient temperature. Fuel volatility is usually expressed by the empirical fuel parameter known as Reid vapor pressure (RVP). These diurnal emissions will occur each day for all vehicles with fuel in the tank, even when stationary.
In an unsealed or open-vented fuel tank, the vapor space is at atmospheric pressure (typically about 14.7 psi), and contains a mixture of fuel vapor and air. At all temperatures below the fuel""s boiling point, the vapor pressure of the fuel is less than atmospheric pressure. This is also called the partial pressure of the fuel vapor. The partial pressure of the air is equal to the difference between atmospheric pressure and the fuel vapor pressure. For example, in an open-vented tank at 60xc2x0 F., the vapor pressure of a typical fuel would be about 4.5 psi. In this example, the partial pressure of the air would be about 10.2 psi . Assuming that the vapor mixture behaved as an ideal gas, then the mole fractions (or volumetric fractions) of fuel vapor and air would be equal to their respective partial pressures divided by the total pressure; thus, the fuel would be 31 percent of the mixture (4.5/14.7) and the air would be 69 percent of the mixture (10.2/14.7).
Diurnal emissions occur when the fuel temperature increases, increasing the equilibrium vapor pressure of the fuel. For example, assume that the fuel in the previous example was heated to 90xc2x0 F., in which case the vapor pressure of that same typical fuel would be about 8.0 psi. To maintain the vapor space at atmospheric pressure, the partial pressure of the air would need to decrease to 6.7 psi, which means that the vapor mixture must expand in volume. This forces some of the fuel-air mixture (i.e. fuel vapor emissions) to be vented out of the tank. When the fuel later cools, the vapor pressure of the fuel decreases, contracting the mixture, and drawing fresh air in through the vent. When the fuel is heated again, another cycle of diurnal emissions occurs.
Evaporative emissions also occur through hot soak loss representing evaporation from the fuel delivery system when a hot engine is turned off and the vehicle is stationary. It arises from transfer of heat from the engine and hot exhaust to the fuel system where fuel is no longer flowing. Other evaporative emissions occur while the vehicle is in motion with the engine operating (commonly referred to as xe2x80x9crunning lossesxe2x80x9d).
Previous attempts to prevent or reduce such emissions have not been entirely successful. For example, evaporative canister systems have been used in an attempt to control evaporative emissions from gasoline-fueled automobiles. The vented fuel vapors are collected in charcoal canisters and intermittently purged by flowing fresh air across the canister while the automobile engine is running and routing the air-fuel mixture to the engine where the fuel vapors are combusted. These systems are complicated and require substantial effort to design. They also require that the engines run frequently to purge the vapors; they do not work well for engines operated infrequently. Pressurized sealed fuel tanks have also been used to contain evaporative emissions, but their design is costly and they raise potential safety concerns. Collapsible fuel bladders have been used to change the volume of the tank in response to changes in fuel volume or vapor pressure, but they are costly and concerns have been raised about their durability. Open-vented fuel tanks have been used for most non-automotive gasoline engine applications. They do not contain evaporative emissions, although substantially prevent pressure build up within the tank.
Accordingly, there is a need for a fuel tank ventilation system and method that substantially prevents evaporative emissions from the fuel tank so as to improve air quality. There is also a need for a fuel tank ventilation system and method of simple construction such that their cost is not prohibitive. There is a still further need for a fuel tank ventilation system and method that are safe, durable, and may be used for engines operated infrequently. There is an additional need for a fuel tank ventilation system and method that substantially prevents emissions with lower pressures than previously achieved with a pressurized tank. The present invention fulfills these needs and provides other related advantages.
The fuel tank ventilation system comprises, generally, a fuel tank and a bladder within the fuel tank that vents to the atmosphere and inflates and deflates with a variable volume of air. The exchange of air between the atmosphere and the bladder substantially maintains the vapor space within the fuel tank at atmospheric pressure and only air from the bladder is vented from the fuel tank because the air within the bladder is isolated from the fuel vapor. The system may further comprise a pressure relief valve and a vacuum relief valve.
The tank may be a sealed fuel tank for receiving and containing liquid fuel including gasoline, methanol or the like. The bladder is preferably constructed of a lightweight flexible material with a low permeability and long-term stability to gasoline and gasoline vapor. The bladder may be sized according to the following equation:             V      B        V    =      1    -                  (                              P            T                    -                      VP            max                          )                    (                              P            T                    -                      VP            min                          )            
Wherein:
VPmin=vapor pressure of the liquid at the minimum temperature.
VPmax=vapor pressure of the liquid at the maximum temperature.
PT=Total pressure (i.e., ambient pressure).
V=total volume of the fuel tank (i.e., fuel capacity plus vapor headspace).
VB=required bladder volume.
The bladder may include sealed pockets for containing air, or other floatation aids to substantially ensure that the bladder remains above the liquid fuel when the bladder is empty.
In an alternative embodiment, the fuel tank ventilation system may be used in combination with a conventional charcoal canister evaporative control system, with the bladder and pressure relief valve being vented through the canister, rather than to the atmosphere.